The present invention relates to managing the number of remote wireless communication units that are performing hand-off, by using directional antennas.
Many types of wireless communications systems such as cellular mobile radio telephone systems and emerging wireless data communication systems provide continuous radio coverage within a geographic area using a number of fixed site transceivers such as base stations or wireless access points. Continuous operation, or at least the appearance thereof, is maintained by automatic transfer, or “hand-off”, of a wireless connection from one site to another as a mobile unit travels around the area.
In urban areas, and in locations with natural obstructions, it is common for radio signals to arrive at a receiver with reflections from nearby objects such as buildings or hills. This can occur along the so-called forward direction, radio path from the fixed cite to the remote unit receivers, as well as in a reverse direction, from the remote units to the fixed site receivers. In either case, the received signal strength can vary substantially as a result of the terrain. Thus hand-off decisions based on simple signal quality measurements do not necessarily follow the same boundaries as the designed cell site boundaries. For example, a remote unit located within the shadow of an obstructing building produces a signal which has significantly decreased, as though that remote unit has indeed traveled outside the cell. A hand-off would therefore be typically initiated by the fixed site. A second hand-off process would again occur once the remote unit moves to a position clear of the obstruction. Indeed, it is as if cell boundaries are rather indistinct.
Additional anomalies occur in other locations, such as at the top of a hill, where a particular mobile unit may be visible to many different base stations. In such position line of site propagation may be available for reliable connections to be made to several base stations causing each such base station to begin to initiate soft hand-off procedures with the single mobile unit.
In digital wireless communications systems, such as those based on Code Division Multiple Access (CDMA) modulation techniques (e.g. IS-95, IS-2000, W-CDMA, and the like), a soft hand-off procedure is used. In this procedure, each mobile unit simultaneously maintains a connection with two to three base stations. As the mobile unit moves from a current cell (source cell) to the next cell (target cell) a traffic channel connection is simultaneously maintained with both cell site base stations. On the forward link, the two signals are combined to yield a composite signal of better reliability. On the reverse link, the mobile unit transmit signal is received by both cell site stations. The two cells demodulate the signals separately, sending the demodulated frames back to a centralized Mobile Switching Center (MSC). The MSC determines the best frame received out of the two that are sent back.
A so-called “softer” hand-off procedure occurs when a mobile unit transitions between two different sectors of the same cell. On the forward link, the mobile performs the same kind of combining process as for soft hand-off. On the reverse link however, the signals from the two sectors are simultaneously received at the same cell site. The signals are demodulated and combined inside the base station associated with that cell, and only one frame is sent back to the MSC.
It is important to note that each sector in a CDMA system is distinguished form the other sectors by a pilot channel code associated with that sector. The pilot channel signals are typically defined by short Pseudo Noise (PN) codes with a specific code phase offset assigned to each sector. A pilot channel serves as a beacon for that sector, and the associated base station aids the mobile in its acquiring other logical channels associated with that same sector.
In both soft hand-off and softer hand-off processing, the mobile unit is an intimate participant in the process. On requirement it has per the above-mentioned standards is to constantly notify the base station regarding its local propagation conditions, by sending Pilot Signal Measurement Messages. The MSC may then make use of this information to make hand-off decisions. This Mobile Assisted Hand-off (MAHO) process requires that the mobile maintain in memory certain lists of base station sectors identifiers and parameters. Sectors are identified in the form of pilot channel identifiers for the sectors. The so-called active set list contains the pilot channel identifiers of those base stations or sectors that are actively communicating with the mobile. If the active set contains more than one pilot channel, then that mobile is maintaining multiple connections on separate traffic channels. In most CDMA systems, the active set can contain up to six pilot channels. A pilot can only be added to the active set if the base stations send a hand-off direction message to the mobile and that message contains a particular pilot channel to be added to the active set.
Soft hand-off capability has been a welcome advance in the field of cellular communication since it encourages the seamless interconnections of calls as a mobile unit travels from sector to sector. However, soft hand-off does not come without a price. While soft hand-off improves the reliability for users in weak signal conditions, such as when they are located on the boundary of a cell or in the shadow behind a building, unfortunately the technique also ends up being used in situations where many signals are available but where connection reliability is not an issue. For example, a mobile unit located on a tall hill may often have good line of sight propagation to several base station sectors. However, using standard hand over processing, each visible sector will be added to the active set, thus tying up traffic channels. This is despite the fact that each individual sector is alone sufficient to maintain reliable communications. However, soft hand-over procedures continue to be utilized due to the fact that more than one sector can be seen by the mobile unit.
In fact, wireless service providers have been making significant efforts towards eliminating excessive soft hand-off where they can find such regions in their networks. They do this by periodically manipulating the point angle of base station antennas, the transmit power of base station power amplifiers and/or the height of antennas in the area adjacent hills. This optimization, which requires significant engineering by human beings, has a goal of increasing capacity by reducing the areas of overlapping signal coverage.
More information on soft hand-over procedures can be found in the book by Yang, S. C. CDMA RF System Engineering (1998 Artec House, Inc., Norwood, Mass.) pages 94–103.